1. Field of the Invention
The present invention relates to video signal processing, more particularly to the detection and reduction of the block noise that often occurs when a digital video signal is transmitted or recorded in a compressed form.
2. Description of the Related Art
Commonly used video data compression algorithms operate on 8×8 blocks of picture elements (pixels). Block noise (also called block distortion) occurs when processing discontinuities at the block boundaries make the blocks visible in the decompressed video picture. Detecting block noise and reducing it by appropriate filtering is a known art.
One known block noise detector, disclosed in PCT Patent Application Publication No. WO2005-004489, employs an edge detector, an edge counter, and a boundary identifier. The edge detector recognizes an edge when the luminance change at a point exceeds the average luminance change at nearby points to the right and left, multiplied by a coefficient. The edge counter has a plurality of counters that count edges detected at different horizontal positions. The boundary identifier receives the edge counts at the end of each video field or frame, at a timing controlled by the vertical synchronizing signal, and detects block noise and the positions of the block boundaries from the occurrence of particularly high counts at intervals of eight picture elements. (See lines 4 to 11 on page 6 of the Japanese PCT publication, and FIGS. 1 to 3).
One problem with this known block noise detector is that it can fail to detect edges at which there is significant luminance variation one side of the edge but not on the other side.
Another problem is that large counters are needed to count edges over an entire video field or frame, and wide data paths are needed to process the resulting counts, making the block noise detector circuit large in size. A related problem is that a large memory is needed to store the video signal for one field or frame awaiting block noise detection, since noise reduction cannot begin until the block boundaries have been detected. A block noise reducer employing this block noise detector therefore has a very large circuit size.